1. Field of the Invention
The present invention relates to a variable valve actuation device for an internal combustion engine, whereby the phase of one of a pair of cams for actuating a pair of intake or exhaust valves is varied relative to the other of the pair of cams by a cam phase change unit.
2. Description of the Related Art
In reciprocating engines (internal combustion engines) mounted on automobiles, a variable valve actuation device is often mounted to the cylinder head of the engine, with a view to improving exhaust gas emission characteristics or pumping loss of the engine.
Such variable valve actuation devices are constructed such that a phase difference between multiple valves (a pair of intake valves or exhaust valves) used in many engines is varied to change the period of time over which the multiple valves are opened. For example, out of a pair of cams for actuating a pair of intake or exhaust valves, respectively, the phase of one cam is varied relative to the other cam.
Many of variable valve actuation devices employ a configuration wherein a shaft member driven by crank output is fitted externally with a fixed first cam and a movable second cam rotatable about the axis of the shaft member such that the first and second cams correspond in position to a pair of intake or exhaust valves, and the phase of the movable second cam is varied relative to the fixed first cam as a reference cam by a cam phase change unit such as a movable vane mechanism, as disclosed in Patent Documents 1 and 2. That is, as the phase of the second cam is varied with reference to the first cam by the cam phase change unit, the period over which the pair of intake or exhaust valves are opened varies greatly. The support stability of the second cam fitted around the shaft member depends upon the width dimension of the second cam, and because of a small clearance provided between the second cam and the shaft member to allow the second cam to rotate relative to the shaft member, the second cam is liable to misalignment due to load applied thereto.
In order to maintain stability of the second cam, a component part having a hollow boss, for example, a cam lobe, is used as the second cam and is fitted at the boss around the shaft member so that the orientation of the second cam may be kept stable.
However, a space above a cylinder of the cylinder head where the first and second cams can be arranged is limited.
Accordingly, in the variable valve actuation devices for varying the phase of one cam relative to the other, the second cam has a one-sided structure such that the boss protrudes not on both sides of the second cam in the width direction of the second cam, but on only one side of the second cam close to the first cam, as disclosed in Patent Documents 1 and 2, in order to maintain stability of the second cam.